DFB lasers are known and are increasingly finding use in applications that require a stable source of electromagnetic radiation of narrow line width, e.g., as source of the signal radiation in optical fiber communication systems.
Prior art DFB lasers generally involve refractive index-coupling, i.e., they comprise a periodic variation in the longitudinal direction of the refractive index of the laser, substantially without periodic variation of the gain or loss of the structure. Such lasers are subject to, inter alia, spatial hole burning, which detracts from their usefulness. The periodically varying region of the laser structure is generally referred to as the "grating".
Loss (or gain; henceforth collectively "loss") coupled DFB lasers are also known. See, for instance, K. David et al., Electronics Letters, Vol. 26, p. 238 (1990). Such lasers comprise a periodic variation in loss in the longitudinal direction of the laser. This is typically expressed in terms of a complex coupling constant .kappa.=(.kappa.'+i.kappa.").
It is generally believed that loss-coupled lasers will exhibit advantageous properties, e.g., reduced spatial hole burning, reduced sensitivity to external reflections, lower frequency chirp, and increased spectral stability. However, in reported loss-coupled lasers the real part .kappa.' of .kappa. typically has substantial magnitude, typically resulting in compromised performance, as compared to the performance expected from lasers that have .kappa.'=0. See, for instance, B. Borchert et al., IEEE Transactions Photonic Technology Letters, Vol. 3(11), p. 955 (1991), which discloses a loss-coupled DFB laser with substantially equal index and gain (loss) coupling strength, namely, about 30/cm; in other words, with .vertline..kappa.'.vertline..about..vertline..kappa.".vertline.. Among other publications of interest is K. Kudo et al., IEEE Journal of Quantum Electronics, Vol. 29(6), p. 1769 (1993), which discloses "complex-coupled" (i.e., gain+index-coupled) lasers with "corrugated" active region. See also J. Kudo et al., IEEE Photonics Technology Letters, Vol. 6(4), p. 482 (1994), which discloses a complex-coupled (including gain-coupled) laser with periodically partitioned active layer, with an identically partitioned "depression layer" on the active layer.
In view of the advantageous characteristics associated with a loss-coupled laser it would be highly desirable to have available a readily manufacturable laser structure that can provide substantially pure loss-coupling. This application discloses such a laser structure.